Method and apparatus for gradually degrading video data

ABSTRACT

A method of transmitting frames of video data to be displayed including the steps of receiving at least a portion of a frame of video data, determining whether the received video data can be stored in a buffer, storing the received video data in a buffer if the received video data can be stored in the buffer, and erasing any video data of the frame of video data if the received video data cannot be stored in the buffer. In addition, an apparatus for transmitting frames of video data to be displayed including an apparatus for receiving at least a portion of a frame of video data, an apparatus for determining whether the received video data can be stored in a buffer, an apparatus for storing the received video data in the buffer if the received video data can be stored in the buffer, and an apparatus for erasing any video data of the frame of video data if the received video data cannot be stored in the buffer.

This is a continuation of application Ser. No. 07/967,487 filed Oct. 23,1992.

RELATED PATENT APPLICATIONS

Related patent applications include copending patent application U.S.Ser. No. 07/967,489, originally filed Oct. 23, 1992 (concurrently withthe present application), entitled "Method and Apparatus for EnablingData Paths On a Remote Bus", hereby incorporated by reference, andcopending patent application U.S. Ser. No. 07/967,488, originally filedOct. 23, 1992 (concurrently with the present application), entitled"Method and Apparatus for Communicating Data Across a Bus Bridge",hereby incorporated by reference;

TECHNICAL FIELD

The present invention relates to handling data transmission overloadsand more particularly to gradual degradation of a multiframe image.

BACKGROUND ART

Computer systems are now displaying a greater variety of graphics, videoand other forms of data concurrently. However, many computer systems donot have sufficient bandwidth resources to update and display all thesetypes of data on a display at the same time.

Some computer systems solve this problem by merely preventing the userfrom requesting more data than can be handled. That is, a computersystem will disallow such overloads in the first place by insuring thatthe peak demand caused by all sources cannot exceed the minimum resourceavailable. For example, the user might be limited in the number of videowindows that may be opened at any given time on a display. However, thissolution does not allow the user to decide how many data sources todisplay in the trade off between the number of sources or windows andthe video display quality. It does not also allow efficient use of theresources available as the demand is not always at peak and the supplyis not always at the minimum.

Other systems have utilized various techniques to degrade each frame ofdata being displayed to lower the bandwidth requirements. For example,U.S. Pat. No. 4,703,439 avoids overloads by varying the resolution of adisplayed image. However, such a technique requires a large amount ofvideo processing to perform the lowered resolution in a visuallypleasing manner.

Another known technique is to vary the frame rate of an incoming videosource. For example, a 16 frame per second video source might bemodified to an 8 frame per second source prior to transmission fordisplay. However, as multiple video sources with different frame ratesmay be displayed on a screen at the same time, it may be difficult todetermine or allocate which video source should have an adjusted framerate. In addition, the overload may be a very temporary overload and theadjustment of frame rate may be unnecessary.

DISCLOSURE OF THE INVENTION

The present invention includes a method of transmitting frames of videodata to be displayed including the steps of receiving at least a portionof a frame of video data, determining whether the received video datacan be stored in a buffer, storing the received video data in a bufferif the received video data can be stored in the buffer, and erasing anyvideo data of the frame of video data if the received video data cannotbe stored in the buffer. In addition, the present invention includes anapparatus for transmitting frames of video data to be displayedincluding an apparatus for receiving at least a portion of a frame ofvideo data, an apparatus for determining whether the received video datacan be stored in a buffer, an apparatus for storing the received videodata in the buffer if the received video data can be stored in thebuffer, and an apparatus for erasing any video data of the frame ofvideo data if the received video data cannot be stored in the buffer.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a high level block diagram of a typical digital computer 100utilized by a preferred embodiment of the invention;

FIG. 2 is a high level block diagram of a first preferred embodiment ofthe inventions, the video transfer channel (VTC), in parallel with amain processor bus;

FIG. 3 is a high level block diagram of a second preferred embodiment ofthe invention, the video transfer channel--stand alone (VTC-SA), with amain processor bus;

FIG. 4 is a timing diagram illustrating a write transfer according to apreferred embodiment of the invention;

FIG. 5 is a timing diagram of a read transfer on a preferred embodimentof the invention;

FIGS. 6A-6G are illustrations of various control fields utilizedaccording to a preferred embodiment of the invention;

FIG. 7 is a block diagram of a preferred graphics adapter and videoadapter utilizing a preferred embodiment of the present invention;

FIG. 8 is a flowchart illustrating a preferred method for graduallydegrading data being displayed;

FIG. 9 is a block diagram of a viewing system according to a preferredembodiment of the invention;

FIGS. 10A-10B are flowcharts illustrating the use of a queueing systemaccording to a preferred embodiment of the invention;

FIG. 11 is a flowchart illustrating disabling or enabling data pathsaccording to a preferred embodiment of the invention; and

FIGS. 12A-12D illustrate data formats according to a preferredembodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a high level block diagram of a typical digital computer 100utilized by a preferred embodiment of the invention. The computerincludes main processor(s) 110 coupled to a memory 120, input device(s)130 and output device(s) 140. Main processor(s) 110 may include a singleprocessor or multiple processors. Input device(s) 130 may include akeyboard, mouse, tablet of other types of input devices. Outputdevice(s) 140 may include a text monitor, plotter or other types ofoutput devices. The main processor may also be coupled to graphicsoutput device(s) 150 such as a graphics display through a graphicsadapter 200. Graphics adapter 200 receives instructions regardinggraphics from main processor 110 on bus 160. Bus 160 may also be used tocommunicate with other adapters. The graphics adapter then executesthose instructions received from the main processor across bus 160 withgraphics adapter processor(s) 220 coupled to a graphics adapter memory230. The graphics processors in the graphics adapter then execute thoseinstructions and update frame buffer(s) 240 based on those instructions.Graphic processors 220 may also include specialized rendering hardwarefor rendering specific types of primitives to be rendered. Framebuffer(s) 240 includes data for every pixel to be displayed on thegraphics output device. A RAMDAC (random access memory digital-to-analogconverter) 250 converts the digital data stored in the frame buffersinto RGB signals to be provided to the graphics display 150 therebyrendering the desired graphics output from the main processor. Thegraphics adapter may also communicate to other adapters across amultimedia bus 260 not directly coupled to main processor bus 160. Thepresent invention is directed to the use of multimedia bus 260

There are two preferred embodiments of multimedia bus 260. Each one ispreferred depending on the characteristics of a desired system. Theseembodiments will be described below as the video transfer channel (VTC)and the video transfer channel--stand alone (VTC-SA). Each of thechannels have some common features and some differences that will alsobe described below.

FIG. 2 is a high level block diagram of the VTC 300 in parallel with amain processor bus 305. The main processor bus is connected to the mainprocessor 315 as well as the graphics adapter 310 and other adapters320, 330 and possible other adapters. The VTC is also coupled to thegraphics adapter 310 and the other adapters 320, 330 and possible otheradapters. The adapters 320 and 330 may be a video compressor, a videodigitizer of an external analog source, a video decompressor, or one ofmany other types of multimedia adapters (such as a MIDI music adapter).In this embodiment, the main processor bus may be coupled to otheradapters not coupled to the VTC. However, in this embodiment, the VTCcan only be coupled to adapters on the main processor bus.

FIG. 3 is a high level block diagram of the VTC-SA 350 with a mainprocessor bus 355. The main processor bus is connected to the mainprocessor 365 as well as the graphics adapter 360. The main processorbus is not coupled to other adapters 370 and 380. The VTC-SA is coupledto the graphics adapter 360 and the multimedia adapters 370, 380 andpossibly to other multimedia adapters. The adapters 370 and 380 may be avideo compressor, a video digitizer of an external analog source, avideo decompressor, or one of many other types of multimedia adapters(such as a MIDI music interface). In this embodiment, the main processorbus may be coupled to other adapters not coupled to the VTC. Inaddition, in this embodiment, the VTC-SA can be coupled to multimediaadapters not on the main processor bus, hence the stand alone nature ofthis embodiment.

The VTC and VTC-SA will be described in greater detail below.

Video Transfer Channel (VTC)

The VTC is a bus intended for high speed communication between adaptersof time-critical data, particularly video display data and possiblyaudio data. The VTC may appear as an adjunct to the Micro Channel orother computer I/O or memory bus, while operating with completelyindependent clocks and controls. Alternatively, the VTC-SA can operateas a self contained bus, including the communication of commands,responses, and compressed multimedia data.

In implementations other than VTC-SA, the VTC is generally not aself-contained computer bus. That is, it generally requires adapters tobe initialized and controlled via a main processor bus or controlchannel such as the Micro Channel (trademark of International BusinessMachines Corporation). Other buses can also be used, such as the PCFamily 1 (ISA) bus or other locally defined buses. VTC carries data andthe minimum information necessary to transfer this data in a direct pathbetween adapters.

Compatibility using the VTC comes at different levels. For plugcompatibility of cards, many aspects besides the VTC itself must becompatible: packaging, connectors, power, cooling, software models, etc.These can be guaranteed by specific implementation details. Chips whichattach to the VTC can be used on different VTC-attached cards if thesame electrical and I/O models are followed. Chips which are differentin these respects can use the same VTC design macros, as the underlyingVTC logical architecture is constant. The VTC provides a differentspecific function from the Auxiliary Video Extension which is found inthe IBM PS/2 (a trademark of International Business MachinesCorporation), and can coexist with it.

The VTC is primarily oriented towards the communications within amachine of uncompressed user presentation information, particularlypixels. It is also capable of carrying other data. Typically the totalI/O requirements of a video adapter are on the order of 1% of theuncompressed video requirements of the same adapter, so in general thereshould be sufficient bandwidth on the VTC for I/O data and controlsignals.

The VTC is an architected interface that can be used for a variety ofpresentation features. The architecture allows compatibility to bemaintained across a full range of systems and features and over a widerange of performance capabilities.

In the preferred embodiment, the VTC is logically a 32 bit bus, withimplicit addressing specified in multiplexed control fields. It ispreferably a multi-master, multi-slave bus with direct support forconcurrent real-time operations. All signals and operations on the busare preferably clock-synchronous. It is designed to require a minimumnumber of signal lines for the function provided. Implementations with16 data bits and 64 data bits are also supported in the preferredembodiment.

The physical and electrical layers are designed for high data ratesconsistent with the characteristics of the technology families assumedin the various product families. As a result the throughput per bussignal line is higher than in many conventional bus designs.

The VTC is preferably a bidirectional data channel that attaches to anumber of devices, typically a display adapter and some number ofprocessing and I/O features. The primary data type for which the VTC isintended is uncompressed digital video or graphics. The VTC providesaccess to the display adapter's display memory and in so doing thetiming of data transfers over the VTC may be independent of alloperational display parameters such as monitor scan rates. The VTC alsoprovides a communication path for video between related devices.

In the preferred embodiment, hardware attached to VTC or the VTC-SA canbe either a master or a slave device. A master device is one thatinitiates transfers of data to or from another device. A typical masterdevice is a source of live digital video. A slave device is one that canbe written or read by a master device. A typical slave device is adisplay adapter.

A single physical device can also operate as both a master and a slave.Display adapter slaves may provide access to their display memory viaVTC. If a slave has multiple layers such as RGB and overlay planes orgraphics and natural image, in the preferred embodiment, these shouldall be accessible over the VTC in order to provide maximumfunctionality. Masters initiate all transfers, by addressing a slave andspecifying the starting addresses and quantity of samples to read orwrite, along with other pertinent information in control fields. Theseare detailed below with reference to Control Fields. In the preferredembodiment, the VTC slaves do not need context memory for the each ofthe individual masters which are interacting with them. Masters supplythe context information necessary for transfers in control fields. Anyadditional context information needed for communications between mastersand slaves is held in the masters. Multiple masters can attach to theVTC simultaneously. Multiple masters interleave their accesses in timeallowing performance equivalent to simultaneous operation of multipleslower channels. Multiple slaves likewise can attach to the VTC. Theyare addressed during VTC transfer operations.

Sharing of the VTC is preferably accomplished through a round robin,non-preemptive hardware arbitration which is modified through the use ofbehavioral parameters. The resulting arbitration behavior is similar tothat of hierarchical multiple round-robin rings. The behavioralparameters and order of the round robin are determined by a schedulingalgorithm preferably implemented in software. The parameters determinedby the software are static for a given configuration of devices anddemands (e.g. windows), therefore there is no real-time requirement onthe software except possibly during dynamic reconfiguration withconcurrent multi-master activity.

Once a master is granted the VTC it preferably retains control until itexplicitly relinquishes it, at which time it passes control (thearbitration token) to the next master in the ring. The behavioralparameters which modify the round-robin action are the maximum amount oftime each master may keep the VTC once it is granted and the minimumdelay since the previous grant before the same master may be grantedagain. A more complete explanation of the arbitration system is givenbelow. The registers for implementing the behavioral parameters inmasters are also described below.

During transfers, such as when a master is sending data to a slave, itmay be possible for data from one master to "broadcast" to multiplyslaves simultaneously. This is implemented by the additional slaves(other than the one which is directly addressed) receiving data beingtransferred on the VTC without driving any signals on the VTC. Theaction has no effect on the operation of the VTC, and is referred toherein as "snooping". A snooping device is responsible for translatingpixel start addresses as necessary and it should not interfere with themaster and slave VTC transfer. That is, it may not drive any lines, inparticular any pacing lines.

While the VTC is preferably used with the Micro Channel (trademark ofInternational Business Machines Corporation) as the base I/O bus, itsuse can be extended to a wide variety of I/O buses for use without anyI/O bus. In so doing the system interface layer of the architecture maychange, due to register mapping, interrupt usage and other reasons. Forexample, entry level PS/2s may choose to implement the VTC in a systemwith the Family 1 (AT) bus. Guest cards (on base cards) which have nodirect access to the Micro Channel or relevant host I/O bus may bedesigned to implement the VTC along with a local I/O bus, including thepossibility of a serial bus (e.g. I-squared-C). In such cases theversions implemented of the physical and electrical layers of thearchitecture may be different from the primary versions specified here.

Video Transfer Channel--Stand Alone (VTC-SA)

VTC-SA provides communications directly to attached video devices, whichmay not connect to any other bus in the computer. With VTC-SA commands,responses to commands, and compressed video data are carried over thebus as well as the normal uncompressed video pixels. The total bandwidthof this additional auxiliary data is essentially insignificant whencompared to the video pixel loading. This configuration decreases pincount for both the graphics adapters and video features, as the fewernumber of pins that are needed simple to provide the video pixelconnections also provide other necessary communications. This additionalauxiliary data is transmitted over the VTC-SA using the same controlmechanism defined here the VTC. The graphics adapter serves as a hostfor the other features and provides a simple and effectivecommunications interface between the system bus and the VTC-SA whileacting as a VTC-SA slave. Certain functions, behaviors and commands aredefined for all VTC-SA devices other than the host to allow consistentinitiation activity from the CPU.

In the preferred embodiment, at any given time there is at most oneactive master and one slave. However, several masters and slaves may betime-domain multiplexed. A master is made the active master throughreceipt and acceptance of the arbitration token, and stops being theactive master by sending out the arbitration token. A slave is made theactive slave by being addressed by the active master. It stops being theactive slave when a different slave is addressed by the active master orthe arbitration token is sent from the active master. The active masterinitiates and performs data transfers with one or more slaves. Alltransfers are initiated, managed and performed using the bus linesdefined below.

The VTC-SA is motivated as a solution to the problem of building videofeatures that have difficulty attaching directly to a general purposeI/O bus such as the Micro Channel. This situation can arise for anynumber of reasons, including packaging, electrical loading, cost, or thelack of such a bus to which to attach. The motivation further includesthe requirement not to add any pins, a primary determinant of cost, tographics adapters, while minimizing or reducing the cost of videofeatures. Performance is also a major consideration, not only for theVTC itself but also that of the system for I/O buses and CPU softwarethat may have to process communications protocols and manage the videofeatures and their data flows among other tasks.

The VTC is more of an adjunct to an existing I/O channel such as theMicro Channel. The VTC would carry uncompressed or raw video pixelsbetween VTC attached devices, including the graphics adapter(s). Such astructure is appropriate in cases where it is assumed that the videofeatures and the display adapter attach to the Micro Channel anyway. Inthat case the VTC provides an optimized path for raw video to passbetween the adapters while providing an architected, consistentinterface for such adapters. In many systems, however, packaging andother architectural constraints change the assumptions used inconstructing the diagram above.

With the VTC-SA, the video features are viewed as parts of the graphicssubsystem and do not attach to the Micro Channel nor any other system orI/O bus. In fact the graphics adapter may not attach to the MicroChannel but rather to a system bus. In that case, even if the videofeatures were to attach to the Micro Channel, a relatively long flexcable would probably be required between the video features and thegraphics adapter, causing unreasonable performance problems. This viewof the video features as part of the graphics adapter system may bereflected in packaging, where video features are guest cards on thegraphics adapter. While it is possible to construct additional buses tosupport auxiliary data flows we are motivated to use the existing pinsand interface logic and the available bandwidth for this purpose.Therefore an architecture is needed that allows video features to existand operate within the graphics adapter system.

For a great many reasons, the use of VTC is already assumed as thepreferred solution to the problem of integrating video features into thegraphics hardware environment. The reasons behind this decision arereadily explained, however they are beyond the scope of this document.

One very significant requirement addressed in the design of VTC-SA isthat of keeping the design of the graphics adapters functions needed toimplement this structure as simple as possible. Some trade-offs are madefor this reason, placing somewhat more than the minimum possible burdenon video features and software. The result is a design that is readilyimplemented in graphics adapters, appearing as a buffered direct memoryaccess (DMA) function, and it is likewise readily implemented in allvideo features and in software. While software has to manage queues, thequeues are consistently architected and software does not have toperform PIO or software-controlled DMA to the video features, generallyresulting in increased performance and greater simplicity when comparedwith I/O mapped or memory mapped video features.

Given that the VTC is in place between the video features and thegraphics adapter, the pins for the VTC are already allocated, the VTCinterface exists and the entire VTC bandwidth is available for whateverpurposes are required by the video subsystem in order to implement theVTC-SA.

Understanding of the data flows that must be supported is required inorder to design a proper connection mechanism, whether or not anadditional bus to the video features is used. Data flows to and from thevideo features generally consist of commands from the host to videofeatures, responses to those commands, compressed data (video) from thehost system to video features, compressed data (video) from videofeatures to the host, uncompressed pixels from video features to thegraphics adapter, uncompressed pixels from the graphics adapter to videofeatures, uncompressed video between video features, and signallinginformation between the graphics adapter and video features(bidirectional).

Not all of these are needed in all video adapters. For example, thecompressed data is obviously only required by compressors anddecompressors. Signalling information between the graphics adapter andvideo features is for tear-free update timing and multi-buffer displaymanagement.

The first two of these data flows, however, commands and responses, arerequired by all features. At a minimum, video features must be able tobe configured and perform diagnostics at POR or configuration time, anduser and screen management controls must be implemented between the CPUand all features.

It is helpful to examine the bandwidth requirements of each of thesedata flows. The main problem addressed by the VTC is the transport ofuncompressed or raw video pixels. The bandwidth for these flows ishighly dependent on the size of displayed windows.

The bandwidth required by any given video window is mostly a function ofthe window size with some exceptions. In the preferred embodiment, thebandwidth is proportional to the area of the window. Bandwidthrequirements can therefore be managed somewhat by managing window sizes.

Bandwidth from a single window is not readily reduced by reducing thenumber of frames per second (fps). To do so would require bufferingentire frames in the feature while also accepting aggravated motionartifacts caused by the slow updates. Multiple windows which might tendto overload the system's capabilities can benefit from dropping someframes or parts of frames as needed dynamically, at the expense ofadditional local memory in the features or degraded image quality.

There may be multiple concurrent data flows of raw video. For example, avideo phone CODEC may display the remote video source in a window of onesize and and the local video source in a different window of a differentsize. In some designs the analog video input circuit may also pass thedigitized local video signal to the compressor over the VTC. A videooutput adapter, for example producing NTSC for recording on tape, may beextracting the contents of a window, including graphic overlays. Allsuch bandwidth requirements for concurrent operations are additive. Someadditional bandwidth is required for command and control messagesbetween the CPU and device attached to the VTC-SA.

There is always a scenario possible which will overload any specifichardware design, i.e. the bandwidth of display memory if not the VTCitself. Therefore software must perform resource management and ensurethat only activities that can be assured to perform properly areactivated. Such assurances can be obtained in a variety of ways,including limiting the size of displayed video windows and allowinggradual degradation of displayed (but normally not digital or analogoutput) windows. Gradual degradation of windows while using the VTC isdescribed herein.

Key features of the preferred embodiment of VTC-SA structure include twounidirectional queues in system memory for auxiliary data OUT to VTC andIN from VTC, the graphics adapter performs first party DMA (bus master)to access these queues, the graphics adapter contains small on-chipbuffers for Out and In data streams, the VTC devices access Out and Inbuffers as VTC masters (graphics adapter is a VTC slave), all auxiliarydata to and from VTC is interleaved into the two system queues using adefined interleaving format, the CPU (or other devices) write and readthe system queues for all communications with video features, and thegraphics adapter contains the pointer registers for the system queuesand on-chip buffers.

VTC and VTC-SA Bus Line Definition

In the preferred embodiments, the VTC and VTC-SA include a 32 bit datachannel plus three control lines and a clock. They are logically 32 bitswide where 16 bit electrical versions serialize the 32 bit informationinto pairs of 16 bit short words. The preferred bit numbering followsthe standard convention, sometimes referred to as "big endian".

The lines of the bus are preferably for a 32 bit implementation asfollows:

DATA(0-31): bidirectional data;

CLOCK (32): continuously running master clock, driven by a singledevice. Input to all master and slave logic;

DATA/-CTL (33): driven by the active master; indicates whether databeing written over the VTC are control fields or data;

MASTERRDY (34): driven active by the active master to indicate that itis presenting valid data during a write transfer, or that it is ready toaccept data during a read transfer;

SLAVERDY (35): driven active by the active slave to indicate that it isready to accept data during a write transfer, or that it is presentingvalid data during a read transfer.

The DATA (0-31) lines are driven by the active master when it isperforming a VTC write transfer, writing control fields, or sending outan arbitration token. The DATA lines are driven by the active slave whena VTC read transfer is in process. The drivers in masters and slaves arepreferably tri-state. The CLOCK (32) line is preferably a continuouslyrunning, constant frequency clock. This signal is used for alloperations of the VTC. All VTC interface logic uses the CLOCK line as aninput for timing. There is preferably one active clock driver. TheDATA/-CTL (33) line indicates whether a master is driving data orcontrol fields and arbitration tokens on the DATA lines. When no masteris driving the DATA lines, or the active master is driving data (e.g.pixel) values, the DATA/-CTL line is in the "DATA" (high) state. Whenthe active master is driving control fields or arbitration tokens on theDATA lines, DATA/-CTL line is in the "CTL" (low) state. Preferredcontrol fields and the arbitration token are described below. Thedrivers are preferably tri-state. There is at least one passive pull-upon the DATA/-CTL line to avoid ambiguities when all masters leave theline tri-stated. MASTERRDY, during write transfers, is driven active toindicate that during the current clock cycle the master is driving validdata on the DATA lines. This data is expected to be received by theactive slave. When the MASTERRDY line is inactive, it indicates thatvalues on the DATA lines are not valid and should be ignored by theslave. During read transfers, the active state of the MASTERRDY lineindicates that data valid during the clock cycle after next will beaccepted, and the inactive state of the MASTERRDY line indicates thatany data present two clock cycles hence will not be accepted and theparticipating slave should not drive valid data during that cycle. DATAline values that are not valid are `don't care`. Preferably the driversare tri-state. There is at least one passive pull-up on MASTERRDY toavoid ambiguities when all masters leave the line tri-stated. TheSLAVERDY line is similarly defined to the MASTERRDY line, with read andwrite, and master and slave reversed. During read transfers it indicatesvalid data being driven onto the DATA lines by the slave. During writetransfers, the active state indicates that data presented during theclock cycle after next will be accepted, and the inactive state of theSLAVERDY line indicates that the active master must not drive valid ontothe DATA lines two clock cycles hence. Again, preferably the drivers aretri-state. There is at least one passive pull-up on SLAVERDY to avoidambiguities when all slaves leave the line tri-stated.

A 16 bit electrical version of the VTC would preferably have the samebus line definitions as indicated above except that there are only 16DATA lines. In the 16 bit version the 32 bit values which would appearon the DATA lines of the full 32 bit version are serialized into pairsof 16 bit values. In the preferred embodiment, operations on the VTCusing 16 bits are serialized versions of 32 bit operations. Data sent toor from 16 bit devices preferably consists of an integer number of 32bit words sent as pairs of 16 bit short words. In such cases the firstshort word contains the most significant 16 bits (0-15) or equivalentlybytes 0 and 1. The second short word from each pair contains the leastsignificant bits (16-31), or equivalently bytes 2 and 3.

Interconnection between 32 bit and 16 bit is possible and does not needto be supported in all 32 bit or 16 bit implementation versions. Where16 bit and 32 bit versions are interconnected the 16 data lines of 16bit devices are attached to the most significant bits (0-15) of the full32 bit VTC. While 32 bit devices serialize all 32 bit words sent to 16bit devices, 32 bit devices may drive all 32 bits when data is directedonly to other 32 bit devices. All 16 bit masters and slaves note thevalues of the 16 MSBs of control fields (data sent with DATA/-CTL "low")to determine whether the information or arbitration token are intendedfor them, and to determine whether the control fields are sent as 32 bitwords or two 16 bit short words. 32 bit masters and slaves use the 16bit form when communicating with 16 bit slaves and masters. As in otheraspects, masters support the characteristics of the slaves with whichthey communicate and do not receive context information from the slavesto indicate these characteristics. 16 bit transfers are indicated by abit in the control fields.

A 64 bit electrical version of the VTC preferably has the same bus linesdefinitions as the 32 bit version except that there are 64 DATA lines.Support for interconnection between 64 bit and 16 bit devices isoptional. If it is supported the requirements are the same as forinterconnected likewise 16 bit slaves and masters as described above.The 64 bit version of the VTC communicates control field information inthe same way as the 32 bit version, with similar considerations forinterconnection with 16 bit devices. 32 bit control words are sent as 32bit words, with DATA(32-63) undefined. Masters and slaves which bothsupport 64 data bits transfer data as pairs of 32 bit words in eachclock cycle. The ordering is such that the word in DATA (0-31) precedesthe word in DATA(32-63). Masters must know which slaves support 64 bittransfers. 64 bit transfers are indicated by a bit in the controlfields.

VTC and VTC-SA Operations

Described below are preferred procedures for Write Transfer, ReadTransfer, Data Transfer, and Pacing.

Write Transfer

With reference to FIG. 4, a write transfer is performed in the preferredembodiments as follows. In a first step, when no transfer is inprogress, MASTERRDY and SLAVERDY are inactive (high). In a second step,the previous active master passes control of the bus to the next masterby transmitting the arbitration token on the data lines to the nextmaster. The token is one of a reserved set of values on the DATA lineswhile DATA/-CTL is low, i.e. a special control field. The previousmaster transmits the arbitration token by driving both MASTERRDY andDATA/-CTL low along with the token value on the DATA lines. The nextmaster receives the arbitration token by noting the `Next Master`sub-field in the Arbitration Token control field being equal to its ownID. No device drives SLAVERDY at this time and its value is ignored byall masters. (SLAVERDY is typically inactive at this time, having beendriven inactive by the previous slave.) In a third step, the masterreceiving the token either initiates a transfer or passes the token onto the next master in the same manner as in the second step. In a fourthstep, the new active master transmits setup information to the chosenslave device in the form of VTC control field values. This informationis transmitted on the DATA lines with DATA/-CTL low and MASTERRDYactive. This setup information includes the address of the targetedslave. In a fifth step, the slave initially is not driving any lines;after it has been addressed via the first control field it drivesSLAVERDY active (low), indicating it is ready to accept more data,assuming that its input FIFO is not full. After the first two words ofcontrol information sent by the master the slave paces the data flowusing SLAVERDY. In a sixth step, once all the necessary control fieldshave been passed to the slave the master clocks data words over the VTC.The master drives MASTERRDY active for each clock cycle in which it isdriving valid data on the DATA lines. When it does not have valid dataready it drives MASTERRDY inactive, causing an idle cycle on the bus. Ina seventh step, control and data words received by the slave are putinto its VTC input FIFO buffer. If the FIFO buffer becomes almost fullthe slave drives SLAVERDY inactive (high) which prevents the master fromwriting the data word two clock cycles later. In an eighth step, theslave stores the data transferred at the addresses in the mannerindicated by the control fields. In a ninth step, once a transfer isinitiated the master and slave must continue processing the transferuntil the transfer is complete. In a tenth step, when the transfer iscomplete the master may either keep the VTC or relinquish it. If themaster keeps the VTC it may make another transfer by writing anynecessary control fields (i.e. at least one, and any which are differentfrom the previous transfer) followed by the necessary data. Itrelinquishes the VTC by sending an arbitration token to the next master.When the transfer is complete the slave drives SLAVERDY inactive andtri-states this line.

Read Transfer

With reference to FIG. 5, a read transfer is performed in the preferredembodiments as follows. A read transfer is similar to a write transferwith these exceptions. First, the control fields written from the masterto the slave indicate that a read transfer is requested. Second, afterthe master has sent the control fields to the slave it drives MASTERRDYhigh, tri-states the DATA lines and waits for the slave to respond withdata. The master waits for SLAVERDY to go inactive for at least onecycle before interpreting the values on DATA as valid returned data. Theslave drives SLAVERDY inactive for at least one cycle before sendingdata. Third, when the slave has data to send to the master in responseto the transfer request, it drives data on the DATA lines and drives theSLAVERDY line active to indicate that valid data is on the DATA lines.Fourth, when the master detects that SLAVERDY is inactive it begins todrive MASTERRDY in accordance with its ability to accept data. The slaveignores the value of MASTERRDY for purposes of pacing for the first twoclock periods after first driving SLAVERDY inactive after receipt of thecontrol fields. Fifth, the slave drives SLAVERDY active whenever it isdriving valid data onto the DATA lines. When it does not have valid datato send it drives SLAVERDY inactive. Sixth, the slave sends a number ofdata items (e.g. pixels) equal to the number specified in the Lengthsub-field in control field F0. Seventh, after the read transfer iscompleted, the master drives MASTERRDY inactive and the slave drivesSLAVERDY inactive. Eighth, the master must detect SLAVERDY inactivebefore driving DATA, DATA/-CTL and the associated MASTERRDY. Ninth, themaster either relinquishes the VTC by sending out the arbitration tokento the next master or it begins another transfer by transmitting atleast one control field.

Data Transfer Procedure

Data passing into or out of a slave is directed to or from its localmemory or registers, with the slave generating all local addresses. Inthe preferred embodiment, the addressing of pixels is in the form ofhorizontal line segments, which can degenerate to a single pixel. Theaddressing of the individual data words or pixels is performedinternally by the slave, based on the start address and otherinformation sent in control fields at the beginning of each transfer.

Before any data are transferred the appropriate control fields must besent to the slave. These fields contain the master and slave IDs,addressing information, the direction of the transfer (read or write),and other information. In some slave implementations some of theinformation necessary to perform a transfer may be contained in localmemory that is indirectly pointed to using the information that istransmitted, i.e. the master device ID or from the transfer Window ID(WID). This fact is transparent to the VTC and all other devices.

The transmission of control fields is similar to data write operations,using the same bus timings, except that DATA/-CTL is driven low by themaster during the clock cycles that control fields are transmitted onthe bus.

The number and size of consecutive transfers that a master may perform,with the same or different slaves, is limited in the preferredembodiment of the architecture only by the arbitration behaviorparameter that limits the maximum length oaf time (number of clockcycles) a master may be granted the VTC. The size of each transferranges from 0 to the maximum value specifiable in control field F0, andagain is limited by the same arbitration behavior parameter.

Pacing

All slaves preferably include an input FIFO function in which data andcontrol information are received. The size of the FIFO is not specifiedby the VTC architecture and is specific to the design of the slave. Allslaves should be able to support the specified pacing mechanism duringall operations, including the transmission of control fields. The totalFIFO size required by each slave depends on such factors as the internallatency and speed of servicing VTC transfer requests.

Data transfers normally occur at the rate of one word for every cycle ofthe VTC master clock. Either the active master or the active slave mayreduce the effective data rate by forcing one or more idle cycles on thebus. This effect is controlled by the use of the MASTERRDY and SLAVERDYlines, which are driven by the active master and slave respectively.This selection of which device drives which of these two lines is thesame for both read and write transfers, however the semantics of thelines changes with the direction of data flow.

During a write transfer, and during the writing of control fields by themaster, the active master drives MASTERRDY active (low) to indicate thatit is driving valid data on the DATA lines. Any time that it is notdriving valid data, such as when it does not have valid data to send, itdrives MASTERRDY inactive. After it has transmitted the arbitrationtoken when relinquishing the VTC, it drives MASTERRDY inactive for onecycle and tri-states this line. Once a slave has been addressed by amaster via the transmission of control information (field F0), the slavedrives SLAVERDY to indicate that the master may or may not continue tosend data during each successive clock cycle. A value of SLAVERDY active(low) during a bus cycle in a write transfer indicates that datatransmitted in the second following cycle will be accepted. A value ofSLAVERDY inactive during a bus cycle indicates that the second followingbus cycle is not to contain valid data and the value on the DATA linesduring that cycle will be ignored.

VTC read transfers are similar to write transfers, except that thedirection of data flow (not control fields) is reversed, and the rolesof MASTERRDY and SLAVERDY are likewise reversed during read datatransfers. The slave drives SLAVERDY active to indicate it is drivingvalid data on the DATA lines, and inactive to indicate a lack of validdata. The master drives MASTERRDY active to indicate that the secondfollowing data word will be accepted, and inactive to indicate that thesecond following bus cycle should be idle with SLAVERDY inactive.

In the preferred embodiment, there are three exceptions to thisdescription. First, when a slave is first addressed by the active mastervia control field F0 it may leave SLAVERDY inactive for two clock cyclesdue to pipeline delays; this applies to both write transfers and thewriting of control fields for read transfers. The master may continue totransmit control fields and data as if SLAVERDY had been active duringthese two clock cycles, and the slave must be capable of receiving andprocessing this data. Starting with the third cycle pacing is asdescribed above. Second, similarly to the above, at the beginning of aread transfer after the control fields have been received by the slavewhen the slave is ready to return the requested data the slave ignoresthe value of MASTERRDY for two clock cycles from the first cycle whenSLAVERDY is brought inactive. The master must be capable of receivingand processing data that may be transmitted from the slave accordingly.Third, arbitration tokens are transmitted regardless of the value ofSLAVERDY. This is necessary because during this time SLAVERDY shouldalways be inactive and tri-stated, and arbitration tokens are receivedby masters (not slaves).

When a read or write transfer is completed the active slave drivesSLAVERDY inactive (high) for one cycle and tri-states this line. At theend of each transfer the active master drives MASTERRDY as describedabove under Write Transfer and Read Transfer.

VTC and VTC-SA Control Fields

The control information sent by a master device to a slave is organizedinto words:called VTC Control Fields. These contain all of theinformation necessary to perform a transfer over the VTC, and are namedF0, F1, F2, etc. These control fields are shown in FIGS. 6A-6G and aredescribed below.

In the preferred embodiment, arbitration tokens are transmitted usingthe same mechanism as control fields. The first (most significant) fourbits of each field specify the identity of the field F0-F14 whosecontents are the remaining 28 bits of contents of the word. The value ofthe 4 MSBs is equal to the numerical portion of the Fn control fieldname. The arbitration token is a special case of a control field and hasa unique number, 15 or `1111`b, indicated in the 4 MSBs.

In the preferred embodiment there is a set of three fields, F0, F1, andF2 which are specified to determine the operation of the VTC. Each timea master is granted the VTC it transmit these three fields to initiate atransfer of pixels. (In the case of slaves which require a start addressof greater than 24 bits F3 must also be written in order to specify the24 MSBs.) F0 must always be written first. It contains the address ofthe slave as well as the master and is needed to ensure that the correctslave receives and responds to the remaining fields and data. If a fieldis not needed for a transfer, e.g., F1 for a transfer that does notcontain pixels, it may be omitted. The other fields may be written inany order. When masters gain control of the VTC they cannot assumeanything about the contents of any fields that they or another devicemay have previously written. Any field values which are necessary inorder to perform a transfer with the addressed slave must be transmittedby the master before any transfers can be performed.

After a master writes the appropriate VTC control fields it begins adata transfer. When the transfer is complete the master may initiateanother transfer. To initiate such successive transfers the master mustsend at least one VTC control field, If the subsequent transfer is to adifferent slave F0 must precede the other control fields. If it is tothe same slave the fields may be in any order. Any field not transmittedwill implicitly retain its previous value. For maximum performance inmaking a large number of short transfers the master may choose totransmit only those fields whose values have changed, e.g. the startaddress.

F0 Control Field

This control field is described below with reference to FIG. 6A. Thecontrol field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0000`b for F0. All bits decoded by allslaves;

bit 4: 32 bit control field. If `1` indicates that this control field ispresented on the VTC as a single 32 bit word. If `0` indicates that thisfield is presented as two consecutive 16 bit half-words;

bit 5: R/-W transfer. If `1` indicates that a read transfer follows; if`0` indicates a write transfer;

bit 6: 32 bit transfer. If `1` indicates that the subsequent transferuses DATA lines 0 to 31. If `0` indicates that DATA lines 0 to 15 areused, and DATA lines 16 to 31 are "don't care". Must be set to `0` in 16bit implementations;

bit 7: 64 bit transfer. If `1` indicates that the subsequent transferuses DATA lines 0 to 63. If `0` indicates that DATA lines 0 to 31 or 0to 15 are used, depending on the value of `32 bit transfer`, and theremaining DATA lines (16-63) or (32-63) are "don't care". Control fieldsnever use DATA lines (32-63), This bit must be set to `0` in 32 bit and16 bit implementations;

bits 8-11: (4 bits) Slave ID. Specifies the 4 bit address of the slavedevice being addressed;

bits 12-15: (4 bits) Master ID. Specifies the 4 bit address of themaster device writing this control field and performing the subsequenttransfer;

bits 16,17: Addressing Mode: Specifies the semantics of the startaddress specified in F2 (and F3):

00: Pixel mode. Start address is a pixel offset from the base of the bitmap window and Length is a pixel count. `Bit map` refers to entirescreen or specific window depending on adapter and windowingenvironment. Relationship between pixel count and word count isindicated by Format;

01: Absolute data mode. Start address is a byte offset from the base ofadapter memory and Length is byte count. There are 4 bytes per data word(up to Length total). The MSBs (bits 0-7) are first. Window ID, FrameBuffer ID, Format, Clip and Stride are ignored in this mode;

10: Reserved; and

11: Register mode. Start address is the base of register address spaceof adapter. Otherwise the same as Absolute data mode; and

bits 18-31: (14 bits) Length. Specifies the length, in pixels or bytes,of the transfer to follow. See Addressing Mode above. Length is anunsigned binary integer, and may range from 0 to 2**14-1. If Length=0,parameters which describe transferred data (Clip, R/-W, Address Mode inF0; 64 bit transfer, 32 bit transfer in F1; Start Address in F2 and F3)are undefined and their values are "don't care".

F1 Control Field

This control field is described below with reference to FIG. 6B. Thecontrol field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0001`b for F1;

bit 4: 32 bit control field. If `1` indicates that this control field ispresented on the VTC as a single 32 bit word. If `0` indicates that thisfield is presented as two consecutive 16 bit half-words;

bit 5: Clip. If `1` indicates that transferred data may contain pixelsthat should not be written to memory and the slave should performclipping internally. If `0` master indicates that any necessary clippinghas been done by the master and slave need not perform any internalclipping. This bit is only defined for Write transfers in Pixeladdressing mode;

bit 6: reserved. Masters must write `0`, slaves must ignore;

bit 7: Stride. If `1` the slave stores transferred pixels to addressesstarting with the Start Address and incrementing by 2. If `0` addressesincrement by 1 pixel per pixel transferred. Defined only for Writetransfers in Pixel mode; otherwise masters must write `0`;

bits 8-15: (8 bits) FB ID: Frame buffer ID, specifies which buffer atthis pixel address is being addressed. Useful for display adapters withmore than one buffer. The value is specific to the slave;

bits 16-23: (8 bits) WID: Window ID, specifies the window ID value usedby the slave; indicates which window is being addressed. The value isspecific to the slave and is not necessarily the same WID value used bythe window management software. May be used directly or indirectly byslave to protect pixels in this or other windows; and

bits 24-31: (8 bits) Format: data format for pixels, including number ofbits per pixel and number of pixels per word. Data formats refer only topixel transfers, i.e. with Addr Mode in F1=0×00. For other addressingmodes the pixel data format is not applicable. Data formats are detailedin greater detail below.

F2 Control Field

This control field is described below with reference to FIG. 6C. Thecontrol field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0010`b for F2;

bits 4: 32 bit control field. If `1` indicates that this control fieldis presented on the VTC as a single 32 bit word. If `0` indicates thatthis field is presented as two consecutive 16 bit half-words;

bits 5-7: reserved. Masters must write `000`, slaves must ignore;

bits 8-31: (24 bits) Start address. In pixels or bytes, according to theAddressing Mode specified in F1, above. If the start address needed forthe transfer is longer than 24 bits, F2 contains the least significant24 bits;

F3 Control Field

In the preferred embodiment, F3 is optional. It is only used whenaddressing slaves that require more than 24 bits to specify the startaddress. This control field is described below with reference to FIG.6D. The control field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0011`b for F3;

bits 4: 32 bit control field. If `1` indicates that this control fieldis presented on the VTC as a single 32 bit word. If `0` indicates thatthis field is presented as two consecutive 16 bit half-words;

bits 5-7: reserved. Masters must write `000`, slaves must ignore; and

bits 8-31: (24 bits) Start address. In pixels or bytes, according to theAddressing Mode specified in F1, above. The most significant 24 bits,for a total of 48 bits of start address, combined with the contents ofF3.

F4 Control Field

F4 is used to signal specific status information between devices. It isnot required to initiate a data transfer. A signalling transfer mayconsist of as little as F0, with Length=0, and F4. Note that the masterin this case may be the same device as a slave addressed in typical datatransfers. This control field is described below with reference to FIG.6E. The control field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0100`b for F4. All bits decoded by allslaves;

bit 4: 32 bit control field. If `1` indicates that this control field ispresented on the VTC as a single 32 bit word. If `0` indicates that thisfield is presented as two consecutive 16 bit half-words;

bit 5: Command: switch to FB ID. If `1` master commands slave to switchWindow ID specified by WID to frame buffer ID specified by FB ID. Thesefields have the same semantics as defined for field F1. "Switch" meansto switch the FB ID used for display or other outpost from the slave,and does not affect addressing and access performed by data transfers.If `0` no command is implied;

bit 6: Status: completed switch to FB ID. If `1` master indicates toslave that frame buffer switch has been completed, with the samesemantics as the command above. Note that the master in this case may besame device as the slave addressed in the command case above, and viceversa. If `0` no status is implied;

bit 7: Status: completed drawing to FB ID. If `1` master indicates thatdrawing has been completed and is temporarily suspended in frame bufferFB ID in area indicated by WID. This is used to indicate to slave thatit may read or write this area (FB ID and WID) without conflict. Notethat the master in this case may be the slave addressed in the commandcase above, and vice versa. If `0` no status is implied;

bits 8-15: (8 bits) FB ID. The ID number of the Frame Buffer indicated.The semantics are the same as for field F1;

bits 16-23: (8 bits) WID. The Window ID number indicated. The semanticsare the same as for field F1; and

bits 24-31: (8 bits) Reserved. Masters must write `0`, slaves mustignore.

F5 Control Field

F5 is used to signal specific timing information between devices. It isnot required to initiate a data transfer. A signalling transfer mayconsist of as little as F0, with Length=0, and F5. Note that the masterin this case may be the same device as a slave addressed in typical datatransfers. This control field is described below with reference to FIG.6F. The control field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`0101`b for F5. All bits decoded by allslaves;

bit 4: 32 bit control field. If `1` indicates that this control field ispresented on the VTC as a single 32 bit word. If `0` indicates that thisfield is presented as two consecutive 16 bit half-words;

bits 5-16: (12 bits) reserved. Masters must write `0`, slaves mustignore;

bit 17: Even/-Odd. If `1` master indicates that `VBI Started` (bit 18)refers to an Even field following in interlaced formats. If `0` masterindicates that an Odd field is referred to. Non-interlaced systems mustwrite `1`. If bit 18 is `0` this bit is not defined and slaves shouldignore;

bit 18: VBI Started. If `1` master indicates that a Vertical BlankingInterval (VBI) of its display function has recently started. The Even orOdd identity of the following field, in interlaced systems, is indicatedby bit 17. If `0` no status is implied;

bit 19: Line started. If `1` master indicates that the display of a linehas recently started. The line referred to is indicated in Line Number,bits (20-31). If `0` no status is implied;

bits 20-31: (12 bits) Line number. Indicates the line number which isreferred to by bit 19 as having recently started display. Numberingstarts at `0` for the first displayed line in non-interlaced systems;

F6-F14 Control Fields

In the preferred embodiment, these control fields are reserved forfuture use. Masters should not use these control fields.

Arbitration Token (F15)

This control field is described below with reference to FIG. 6F. Thecontrol field bits are preferably as follows:

bits 0-3: (4 bits) Field ID=`1111`b for Arbitration Token (F15);

bits 4: 32 bit control field. If `1` indicates that this control fieldis presented on the VTC as a single 32 bit word. If `0` indicates thatthis field is presented as two consecutive 16 bit half-words;

bits 5-7: reserved. Masters must write `000`, slaves must ignore;

bits 8-15: (8 bits) Next Master ID. Bits (12-15) are the ID of master towhich the arbitration token is sent. Bits (8-11) are reserved and mustbe 0. Devices must decode all 8 bits including bits (8-11)=`0000`;

bits 16-23: (8 bits) Previous Master ID. Bits (20-23) are the ID of themaster sending the arbitration token. Bits (16-19) are reserved and mustbe 0. Devices must decode all 8 bits including bits (16-19)=`0000`; and

bits 24-31: reserved, masters must send 0, slaves should ignore.

VTC and VTC-SA Arbitration

The arbitration function of the VTC is a round robin that is modified bybehavioral parameters. The round robin is performed by a token-passingmechanism. Whenever the currently active VTC master device relinquishesthe bus it transmits the arbitration token to another master called NextReady in this document. The recipient, or destination, of the token is amaster whose ID is specified by the master passing the token. The valuesof the token destinations in each of the masters is arranged, bysoftware, such that all masters which are operational are in a logicalring, and the token passes through all such masters once each cyclethrough the ring. In alternative implementations the token may be passedto different masters according to the scheduling algorithm implemented.

If a master receiving the token does not need to use the VTC, or it hasbeen prohibited from using it at that moment (via arbitration behaviorparameters), then it passes the token to the next master in the logicalring. Initialization of the arbitration token is accomplished after eachdevice is tested and programmed with its token destination andscheduling behavior. At that time, with no token in the system, anymaster device can be directed by software to transmit the arbitrationtoken, thereby initializing the arbitration token logical ringmechanism. Any time-out exception condition detection functions in VTCdevices are disabled prior to initialization.

In VTC-SA, the host (graphics) adapter initiates sending the token toeach of the possible VTC devices to allow each device to respond withinitialization confirmation. Further initialization activity in eachdevice requires the device to receive data directed to it from the CPUover the VTC, and responses are returned using the same mechanism overthe VTC.

The behavioral parameters which modify the round-robin action are themaximum amount of time each master may keep the VTC once it is grantedand the minimum delay since the previous grant before the same mastermay be granted again. These amounts of time are specified in terms ofnumbers of cycles of the VTC Clock signal. They are specified inregisters in each master device.

Limitations on arbitration behavior are imposed by behavioral rulesestablished by software or by device design. There is no inherentelectrical or physical limit in the VTC as to how long a master devicemay retain control of the VTC.

In the preferred embodiment, active masters and slaves must take certainactions within specified time limits of certain events occurring. Alllimitations are in units of cycles of the Clock signal. First, uponreceipt of an arbitration token a master must either initiate a transferwithin for example (2) cycles or pass the token to the next masterwithin e.g. (2) cycles. Second, upon completion of a transfer the activemaster must transmit the token or begin another transfer within e.g. (2)cycles. Third, upon being addressed via field F0 the active slave mustrespond with SLAVERDY active within e.g. (10) cycles.

Some possible exception conditions may require attention. First, amaster not initiating a transfer or passing the token upon receipt ofthe token. Second, a master failing to transmit the token when it hascompleted a transfer. Third, a master sending a token when a transferhas not been completed. Fourth, a master sending a token when it is notthe active master. Fifth, a slave not responding to being addressed viaF0 in either a read or write transfer. Sixth, incorrect data or controlfield values being received by a slave. Seventh, actual transfer lengthnot in agreement with the Length field of F0. Eighth, values received onthe VTC bus not the same as were transmitted.

Any master or slave may implement detection of some exceptionconditions. Conditions which can be detected by devices which may not beparticipating in a transfer or token-passing include the following.First, an arbitration token being transmitted and the next master notresponding within the preferred time limit of clock cycles. Second, noarbitration token being transmitted while MASTERRDY is inactive for morethan the preferred limit of clock cycles. Third, data transmitted beforecontrol fields are transmitted after an arbitration token istransmitted. Fourth, any arbitration token being transmitted where the`Previous` master is not the same as the previous `Next` master. Fifth,a slave not driving SLAVERDY active within the preferred time limit ofclock cycles after it was addressed via F0. Sixth, SLAVERDY inactive formore than the preferred limit of clock cycles during a transfer (before`Length` items have been transferred). Seventh, more than `Length` dataitems produced (with SLAVERDY active) during any one transfer.

Graphics Adapter

Graphics adapters which act as hosts to VTC-SA attached devices mustperform certain functions in specified ways in order to implement thepreferred embodiment of VTC-SA. The graphics adapter must provide accessto its display buffer via the VTC in addition to the function specified.The function of accessing the display buffer is not described here.Typically the graphics adapter acts as a VTC slave for accessing thedisplay buffer.

Local Buffers

The Out buffer in main memory serves to store data obtained from systemmemory over the system bus before the data is obtained by VTC devicesover the VTC. The In buffer in main memory serves to store data sentfrom VTC devices over the VTC before it is written to system memory overthe system bus. Each of these buffers is preferably at least 32 bytes (8words) in length. In principal smaller buffers are possible, howeversystem performances would suffer with smaller buffers, so they are notrecommended. While the In and Out buffers can each be accessed by twodifferent processes, the bus interface and the VTC interface, they arenot required to support simultaneous dual port read-write access.

Each of these two buffers has a Count register associated with it. TheOut Count register indicates the amount of data available in the Outbuffer. The In Count register indicates the amount of space availablefor data in the In buffer. Both of these registers are readable via theVTC. The buffer addressing may be implemented in a number of ways, TheCount register may be implemented to contain the actual current addressin a single ported buffer, or it may contain the difference betweenindependent Read and Write pointers.

System Bus Interface

In the intended applications of VTC-SA, the graphics adapter is a busmaster on the CPU bus. The graphics adapter has access to the two systemqueues dedicated to supporting VTC-SA. As such, the graphics adapter hastwo sets of pointers on board which point to the queues. Typically thesepointers are for the Top, Bottom, Read, and Write positions of eachqueue, requiring four registers per queue. The register sizes depend onthe system design. The pointers use real memory addresses.

With the system queue pointers on the graphics adapter, the graphicsadapter can readily identify the amount of data and space available ineach queue. Software can also determine the amount of space and dataavailable in each queue by examining these pointer registers.

The graphics adapter should have a programmable capability of disablingqueue bus master activity. This is needed so that the system canre-direct the system queue pointers without inadvertently indicating tothe graphics adapter that bus master activity is required, which couldpotentially access an incorrect address range.

In the preferred embodiment, the implementation is simplified if alldata in queues to support VTC-SA consist of integral multiples of 32bits (one word) and are aligned on whole word boundaries, equivalentlyon byte addresses which are integral multiples of 4. The graphicsadapter can rely on data following this constraint in both system queuesfor VTC-SA, and therefore does not need to perform byte rotations, etc.,which might otherwise be needed if arbitrary addresses were allowed.

With the graphics adapter being a system bus master and using busmastership to maintain the VTC-SA queues and buffers, it supports twoprocesses for the purposes of VTC-SA in addition to whatever otherprocesses it may be designed to support. These include attempting tokeep the Out buffer full and the associated Out queue empty andattempting to keep the In buffer empty and the associated In queue full.

Procedurally, the graphics adapter manages the system queues and VTC-SAon-board buffers as follows. Whenever the Out buffer has space availableand the Out queue has data available, the graphics adapter attempts toarbitrate for the system bus. When arbitration is granted, the graphicsadapter transfers from the queue to the buffer the lesser of the amountof space in the buffer and the amount of data in the queue, up to thelimits of the bus grant period.

Whenever the In buffer has data available and the In queue has spaceavailable, the graphics adapter attempts to arbitrate for the systembus. When arbitration is granted, the graphics adapter transfers fromthe buffer to the queue the lesser of the amount of data in the bufferand the amount of space in the queue, up to the limits of the bus grantperiod.

If the buffers are single ported, then at the time the system bus isgranted if the desired buffer is busy with VTC activity the graphicsadapter may have to terminate or pace the bus activity, depending on thedesign of the bus and the bus interface circuitry.

Upon the completion of reading data from the Out queue, the Out queue'sRead pointer is updated by the graphics adapter. The CPU can therebydetermine the new amount of space available in the Out queue.Simultaneously, the graphics adapter updates the count of data availableassociated with the Out buffer, allowing VTC devices to determine theamount of data available in this buffer.

Upon the completion of writing data to the In queue, the In queue'sWrite pointer is updated by the graphics adapter. The CPU can therebydetermine the new amount of data available in the In queue.Simultaneously, the graphics adapter updates the count of spaceavailable associated with the In buffer, allowing VTC devices todetermine the amount of space available in this buffer.

Determination of whether there is data available in the In buffer orspace available in the Out buffer, such that system bus activity isrequired, is based on the Count register or other circuit providing asimilar function. This determination should be made after the completionof any active VTC transfers to or from the In or Out buffersrespectively. The reason for this is that otherwise the system busmaster may transfer an amount of data less than a normal burst becausethe estimate of the amount to transfer was made prematurely, resultingin inefficient use of the system bus, and possibly inefficient use ofthe VTC and increased latency as well.

For example, if the In buffer is initially empty and a VTC device beginsa transfer of 32 bytes (8 words) the graphics adapter's bus mastershould not start a transfer until the all 8 words are in the In buffer.This result is assured if the bus arbitration does not begin until theVTC transfer is complete.

At the same time there are important cases where the amount of data tobe transferred over the system bus or over the VTC is less than 8words--data streams in multiples of e.g. 32 bytes cannot be guaranteed.When current VTC transfers to and from the buffers have been completedthe system bus interface must process the complete contents of thebuffers without waiting for further VTC transfers even if that resultsin system bus transfers that are shorter than normal bursts. Otherwiseunacceptable latencies of small amounts of important data could occur.

The graphics adapter may have multiple internal tasks vying for use ofthe bus mastership capability, and therefore an arbitration scheme isneeded. The VTC-SA queue and buffer management processes should be thehighest priority tasks within the graphics adapter's bus mastership.This is a result of the fact that the VTC auxiliary data are normallyreal-time with deadlines that must be met in order to avoid bufferoverflows or underflows and frequent servicing is necessary. Likewise,the graphics adapter should have a high priority on the CPU bus or MicroChannel to support guaranteed timely servicing. Again, this is a resultof the need for frequent servicing.

Similarly, software that manages the system queues must executefrequently and have a high priority or frequent guaranteed scheduling.

The need for interrupts from the graphics adapter to the CPU is notgenerally established. This is a system-dependent performance issue andis beyond the scope of this general architecture. Interrupts, e.g.signalling that the Out queue is empty or that the In queue is full, maybe useful in some software architectures in order to guarantee timelyservicing of queues.

Queues which utilize a level of indirection by containing pointers tosystem data buffers may be implemented. This has no effect on the VTCinterface.

VTC Interface

In the preferred embodiment, in transfers where auxiliary data arepassed between the graphics adapter and VTC devices the graphics adapteracts as a VTC slave.

To retrieve data from the Out buffer, VTC devices act as masters. Beforeretrieving data, the VTC device reads the Out Count register via the VTCto determine how much data is available in the Out buffer. It may then(or later) perform a Read transfer over the VTC, from the graphicsadapter, of up the amount of data specified by the Out Count registervalue received. The Out buffer is treated as a FIFO by the VTC device,i.e. a fixed start address is used for such transfers.

Sending inbound data is similar. That is, the VTC device masters the VTCand reads In Count register from the graphics adapter. It maysubsequently perform a Write transfer to the graphics adapter of up tothe amount data space indicated in the In Count value received. As inOut transfers, a fixed address is used for In buffer Write transfers.

The Count registers for the In and Out buffers are unsigned binaryintegers which indicate a number of 32-bit words or data or space asappropriate. They each occupy a 16-bit field within a single VTC word.

Typically the graphics adapter will require only 4 to 6 bits forsignificant information in the Count registers. The more significant,constant bits should be filled with zeros. Note that for e.g. an 8 wordbuffer there are 9 possible values, 0 through 8, requiring 4 bits.

Note that while the Count fields indicate a number of 32-bit words, theVTC transfers in Memory and Register addressing modes specify the lengthas a count of bytes. The byte count of VTC may be useful for otherapplications. VTC devices performing auxiliary data transfers accountfor the difference here, a factor of 4, when converting from the Lengthfields above to the length field in the VTC F0 control word.

Two addresses are specified over the VTC to perform the above functionsof In and Out Count registers and In and Out buffers. The In Count andOut Count registers are accessed as one Read transfer of the Count fieldas above. One embodiment is to map the Count registers at address mode`10`, currently reserved, and start address `0`. The In and Out buffersare accessed using the same address. Out uses a Read transfer and Inuses a Write transfer. Another embodiment is to map the In and Outbuffers to address mode `11`, Register mode, and start address `0`. Ifboth of the above embodiments are used then it would be possible toignore the start address field (F2) of the VTC altogether during suchtransfers, minimizing overhead on the VTC.

The graphics adapter requires very rudimentary VTC master capabilitiesto implement a VTC-SA host adapter. These are: send the ArbitrationToken to a designated VTC ID, continually resend the Arb token, whenreceived, to the same VTC ID, or stop the VTC Arb token when it isreceived and indicate to the system that the token has been received.These capabilities allow the system to determine which VTC IDs haveinstalled devices, and allows initializing all VTC devices. In steadystate operation, after initialization, the graphics adapter continuallyresends the token to a designated ID every time the token is received,without performing any other VTC master activity. VTC mastercapabilities may be included in graphics adapters if they are needed forother reasons beyond the requirements of VTC-SA.

While the VTC inherently allows pacing of all transfers by both mastersand slaves, specific performance requirements are included for auxiliarydata transfers in order to maintain bus efficiency and systemperformance.

Reads of the Count registers should be completed as quickly aspractical. VTC clock cycles may pass with the graphics adapter holdingthe VTC line SLAVERDY in the non-ready state before the Count data valueis actively placed on the VTC. Read transfers from the Out buffer mayhave not-ready VTC clock cycles before beginning to place activereturned data on the VTC. Subsequently the requested Read transfer datashould be transferred at the rate of one word per two clock cycles orfaster. For 16 bit VTC this means one half-word every clock cycle. For32 bit VTC this means one word on cycles than may alternate with at mostone non ready cycle. Write transfer to the In buffer timing requirementsare similar: The VTC master device may begin writing data immediatelyafter writing the VTC control fields. The graphics adapter (VTC slave)may cause a number of not ready cycles; these should be limited to thenumber of words to be transferred plus e.g. 3 during each such transfer.Exceptions to the above may have to be made in cases where the graphicsadapter's VTC FIFO or FIFOs are already occupied with recent previousVTC transfers which access the display buffer. The graphics adapter canassume an interval exists between successive reads of the Countregisters that is greater than or equal to a specified value.

Video Feature Functions

VTC attached video features perform all data communications with thehost system via the outbound and inbound data paths, consisting of theOut queue and buffer and the In queue and buffer. In the following thereis reference to and dependency on data formats which apply to theauxiliary data streamed to and from the VTC devices.

The VTC-SA architecture performs well for configurations with one ormore VTC devices attached. To simplify the explanation the situationwith only one device is described first. A later section specifies thedevice requirements and behavior for use with multiple devices.

The VTC device polls the graphics adapter to determine the amount ofdata in the Out buffer and the amount of space in the In buffer. Ifthere is data available in the Out buffer and the VTC device has spaceto accept this data it performs a VTC Read transfer and reads this data.The device may read less than the available amount of data if less spaceis available in internal buffers. Good system design should ensure thatthe VTC device will always have space available for the data, but thatfact is not guaranteed by this architecture as it depends on a largevariety of factors beyond the scope of this architecture.

The VTC device processes the received data as appropriate according tothe data type. This includes determining which data constitutes commandsand which is compressed data, such as video, and then processing thecommand or compressed data accordingly.

If the VTC device has data to send to the system it polls the graphicsadapter to determine the amount of space available in the In buffer. Itsubsequently transfers the lesser of the amount of data waiting to besent and the amount of space available in the In buffer to the In buffervia the VTC, using a VTC Write transfer.

In many cases it should not be necessary to support more than one VTCattached feature simultaneously. Therefore the functions listed belowfor multiple VTC devices may not need to be implemented for adaptersintended for such systems.

If there is more than one VTC feature attached, one is designated duringinitialization as the Distributor, and all other devices are referred toas Alternates. The Distributor handles all outbound data, which ispassed on to the Alternates as appropriate. Alternates move inbound datadirectly into the In buffer (and therefore to the In queue) under thecontrol of the Distributor. The Distributor behaves exactly as a soleVTC device with the additions of routing outbound data to Alternatedevices and controlling inbound data transfers from Alternate deviceswhen necessary.

All VTC devices initially operate as if they are sole VTC devices whenthey are first initialized. Indeed in the preferred embodiment, this isnecessary as such devices cannot be told to operate differently untilthey have received commands from the CPU.

If some of the outbound data received by the Distributor in a readtransfer is the header of a packet intended for a different (Alternate)device the protocol is as follows. First, the Distributor passes amessage to the target Alternate device, indicating the existence andlength of the packet to be retrieved (it is not necessary for the entirepacket to be held by the Distributor initially). Second, the Alternatedevice reads the data from the Distributor. Repeated reads occur asneeded until the complete packet has been read. Third, the Alternatesends a message to the Distributor indicating completion of receivingthe packet.

In transferring the packet from the Distributor to the Alternate theDistributor behaves the same way as the graphics adapter does fortransferring auxiliary data to VTC devices. By examining the destinationand length fields encoded in the data stream, however, the Distributorensures that only data intended for the Alternate is sent to it, andlikewise the Alternate is ensured to read only the amount of data thatis intended for it. Typically the Distributor will include a separatedata buffer for use in resending auxiliary data to other VTC devices.

Redirecting packets to alternate devices may involve some latency in theDistributor. That is, the parsing and redirection could involve theassistance of an on-board processor as long as there is sufficientbuffer capability and the total latency does not cause any system-levelproblems. Processor-assisted redirection should normally not incur morethan about 10 us additional latency with a simple design. Additionallatency on the order of 100 us to 1 ms should not normally cause aproblem.

If there are multiple VTC devices installed and a device (Alternate)other than the Distributor has data to send to the system the protocolis as follows. First, the Alternate device sends a message to theDistributor requesting use of the In buffer. Second, after theDistributor has completed sending its current packet the Distributorreturns a message to the Alternate indicating that the In buffer is nowowned by the Alternate device. Third, the Alternate device then behavesexactly as the distributor and writes data to the In buffer according tothe amount of data and space that is available. Fourth, upon completionof the Alternate device's writing of data to the In buffer the Alternatesends message to the Distributor returning control of the In buffer.While the Alternate is using the In buffer no other device, includingthe Distributor, may write data to the In buffer. The exception to thisrule is when the error condition occurs of the Alternate violating thetime limit for ownership of the In queue. In that case the Distributorsends a Preempt message to the Alternate and then begins writing packetsto the In buffer. Presumably such packets indicate the fact that theerror was detected. When such an error and recovery occur, the system'sIn queue will probably have the next packet Flag in a location notpointed to by the previous Length field in the Descriptor word. Theparsing software will have to recover by finding the new Flag word andverifying via the Length and the subsequent Flag word.

Note that the Distributor cannot give ownership of the In buffer to therequesting Alternate until it has completed sending its current packet.The amount of time this requires depends on the packet size, frequencyand latency of accessing the In buffer, latency of emptying the Inbuffer to the In queue by the graphics adapter, and whether there isspace in the In queue. Also, if more than one Alternate requests use ofthe In buffer they must be given access in some order, so someAlternates may have an additional wait. For all these reasons,guarantees as to the latency of Alternates obtaining the In buffer arebeyond the scope of this specification unless performance of the entiresystem is covered, including the software which parses the In queue.

Data communicated between the system and any VTC device (not just theDistributor) are in packets. The packet sizes are not necessarilyrelated to the sizes of the system queues or the In and Out buffers, andtherefore are not necessarily related to the sizes of VTC datatransfers. In the descriptions above for distributing data to alternatedevices and concentrating data from alternate devices these data packetsdetermine the amount of data to be retrieved from the Distributor orsent from the Alternate to the graphics adapter, respectively. Whenthere are multiple VTC devices, Alternate devices are directed toretrieve only whole packets. Likewise, when Alternate devices arewriting data to the In queue only whole packets may be written duringthe interval that the Alternate device controls the In queue. It is notnecessary, however, and generally not practical for packets to becommunicated in a single VTC transfer.

Bus Initialization

In the preferred embodiment VTC and VTC-SA devices power up in aquiescent state on the VTC. Devices power up with a VTC device ID thatis a function of design and the slot wiring. For example, a class ofsystems may exist for which there is a maximum possible of 4 VTC devicesinstalled. Each device would then be designed via either chip design orcard wiring to have all but 2 ID bits fixed, and those two other bitswired to the VTC connector. Each connector, up to a maximum of 4, wouldhave a unique setting for those two bits.

The graphics adapter which hosts the VTC devices is preferably hardwired to a fixed VTC ID. The ID of the graphics adapter is designed intoall VTC-SA devices as the device which they communicate with uponpower-up. The CPU tells the graphics adapter to send the VTC token toeach possible VTC ID in turn to see whether the token is returned,thereby detecting the presence of VTC devices. To initialize a devicethe CPU puts initialization commands in the Out queue, sets up the Outqueue in the GA, enables the Out queue, and tells the graphics adapterto send the token to that specific VTC device continually. All VTCdevices power up with a fixed initial Next Master ID for use in sendingthe token, and that ID is the same as is used by all hosting graphicsadapters. The indicated VTC device then polls the graphics adapter forthe presence of data in the Out buffer and subsequently reads thepackets of data from the CPU via the Out buffer. When it has a responseavailable it returns the response via the In buffer. To initialize otherdevices the CPU tells the graphics adapter to cease sending the token tothe first device and the operation above is repeated with the eachadditional device.

Once all devices are initialized, if there are more than one, one deviceis designated as the Distributor and all other devices are designated asAlternates. Each device is set via its Next Master value to send thearbitration token to a designated device, thereby configuring alldevices in a round robin.

If there are multiple devices, the Alternates are commanded not to querythe Out queue or send data to the In queue except as described herein.For devices other than the Distributor the Out queue exists in theDistributor, and the In queue may only be used when it is granted by theDistributor.

Gradual Degradation

The present invention is directed to gradual degradation that operatesas needed in overload situations providing effective utilization of thebandwidth of the VTC or VTC-SA while allowing the user to determine thebalance between number and type of sources and the display quality. FIG.7 illustrates a video adapter utilizing a preferred embodiment of thepresent invention in combination with a graphics adapter for displayingthe video. Video adapter 600 receives a video input signal on videoinput bus 605 which may be a NTSC analog signal the like. Adapterprocessor 615 receives and digitizes the analog video input signal andstores the results of the digitization in a buffer 620 prior totransmitting the data on bus 610 which may be the VTC, VTC-SA or othertype of bus. In a preferred embodiment, the buffer is a circular bufferwith a head pointer 625 and a tail 630 pointer to indicate where thevideo data is stored in the buffer. As the incoming analog data isdigitized, the data is stored in buffer 620 and the adapter processor615 updates the head pointer. As the bus allows, the adapter processorreads data from buffer 620, updates tail pointer 625 and transmits thedata to the graphics processor on bus 610. In the preferred embodiment,the buffer 620 is a circular memory and will hold approximately oneframe of video data. Graphics adapter 650 includes a graphics processor655 which receives the data from video adapter 600 plus from othersources on the video bus and loads that video data into frame buffer 660for display on display 665.

FIG. 8 is a flowchart illustrating a preferred method of graduallydegrading data being displayed. In a first step 700, adapter processor615 receives analog video input data on video input bus 605 anddigitizes that data for eventual display. In step 705, the video adapterdetermines whether the buffer 620 is full by comparing the head and tailpointers to see if the head pointer has wrapped around to the tailpointer. If the frame buffer is not full, then in step 710, the adapterprocessor loads the digitized data into the buffer 620 and then updatesthe tail pointer 630. This process continues between steps 700, 705 and710 while the adapter processor is also transmitting data from thebuffer to the graphics adapter across bus 610.

If the buffer 620 is full indicating that the video adapter has beenunable to transmit previously stored digitized data to the graphicsadapter, then processing continues to step 715. In step 715, the adapterprocessor erases the current frame being stored in the buffer. Theadapter processor accomplishes this by merely moving the tail pointer tothe end of the previous frame or portion thereof stored in the buffer.In step 720, the adapter processor then throws away the data justdigitized. In step 725, the adapter processor then receives more analogdata and may digitize that data. In step 730, if the latest digitizeddata is from the same frame that has been thrown away, then processingcontinues to 720 where that new frame data is then again thrown away.However, if the recently digitized data is from a new frame, thenprocessing returns to step 705 where the adapter processor againdetermines whether the buffer is full. Under most circumstances, thebuffer should have plenty of room for starting to enter another videoframe.

One of the advantages to the preferred gradual degradation technique isthat the video frames or other types of groups of data are thrown awayonly as needed. That is, it is only when the video adapter is unable tostore and transmit all incoming data that any frames are thrown away. Inaddition, if there is any sort of prioritization scheme wherein variousadapters may have greater priority, a video adapter having less prioritywill generally have more frames thrown away should an overload conditionoccur.

System Queues

In the preferred embodiment there are two system queues for VTC-SA, theOut queue and the In queue. More queues may be implemented. The actualaddresses of these queues as seen by hardware may be changed dynamicallyunder software control. However, there is still one queue in eachdirection. Dynamic positioning of the queue pointers may provide datagathering capabilities. The two queues are illustrated in main memory inFIG. 9. All addresses are preferably on whole word (32 bit) boundaries,or equivalently, at integer multiples of 4. In the preferred embodiment,fractions of words are not transmitted or received for simplicity.

The Top and Bottom pointers are fixed while each queue is active,although they may be changed under some circumstances while the queuesare disabled. The Read and Write pointers are used and updated by theprocess (system or graphics adapter) that is reading or writing datafrom or to the queues, respectively.

Data in the queues is organized into packets. For outgoing data, thesystem builds complete packets, one following the other, and places themin the Out queue. Each packet begins with a Flag word to aid parsing anda Descriptor word containing the length and destination (VTC device ID)of the packet as well as an indicator that the data contents are acommand or data. The details of these words are contained under "DataFormats". Any mixture of data and commands to any VTC device may beinterleaved in the single Out queue using this mechanism.

All data in the Out queue are in complete packets, with the Flag wordand correctly-valued Descriptor word. This restriction applies to alldata including the first data placed in the Out queue duringinitialization. This allows reliable and fault tolerant parsing of thedata by VTC devices. This restriction does not mean that completepackets must be placed in the queue at one time. Portions of a packetmay be added until the packet is complete.

All data in the In queue is constrained similarly to that of the Outqueue. It consists of complete packets. Each packet starts with a Flagword followed by a Descriptor word, which indicates length, VTC sourcedevice, and Response or Data.

All data placed in the In queue by any VTC device consists of completepackets. This restriction applies to all data, including the first dataplaced in the queue during initialization, and even if data originatefrom multiple VTC devices. This restriction supports reliable andfault-tolerant parsing of the In queue by software. As with the Outqueue, this restriction does not require complete packets to be placedin the queue at one time. Portions of a packet may be added until thepacket is complete.

The parsing task will find the Flag as the very first word in the Inqueue, and the Length is an offset pointing to the next Flag. In thisway reliable parsing is assured. The source ID and response or dataindicator may be used to direct the data to any other task or routinethat must process the data in the In queue. Data may be directed byreference through the use of pointers if multiple areas of memory areused for the In queue.

In systems that are virtual memory with e.g. 4 kB pages, each queue canbe larger than one page by including in the design a separate table ofthe page base addresses in order. The table should be accessible by boththe graphics adapter and the CPU, and its values should be fixed.

The In queue can be remapped at any time with the queue temporarilydisabled as long as the data which is not parsed yet is retained and theparsing task completes parsing of all data in order.

Unlike the Out queue, if the In queue is dynamically moved using thequeue pointers, in general it is not possible to guarantee completepackets in each distinct region of memory. VTC devices and the graphicsadapter will put data in the In queue as needed and in general memoryaccesses by the graphics adapter will not align with packet end points.Ensuring that each distinct region of memory used for the In queuecontains only complete packets can be accomplished if the software movesdata between regions of memory as necessary after the data has beenreceived from the graphics adapter.

Queues may be implemented as sets of pointers to additional data buffersin system memory. There may be advantages in terms of flexibility andsystem performance. Whether or not indirect queues are implemented hasno effect on the VTC interface, Indirect queues constitute asystem-dependent option.

Queues may also be implemented as a mixture of indirect and immediatedata, e.g. the main queue contains packets consisting of a command word,indicating immediate or indirect (it also serves for fault-tolerantparsing), a pointer word (this exists only for indirect data), a lengthword, indicating the length of the actual data, whether indirect orimmediate, and immediate data, in the case that immediate mode is usedfor this packet.

FIG. 9, illustrates a preferred embodiment of a queueing system forbridging messages or data between the main processor and adapters on theVTC-SA bus. Main processor 400 is coupled to main memory 410 whichincludes an In queue 412 and an Out queue 414. The main processor isalso coupled to the graphics processor 425 of graphics adapter 420across system bus 418. The graphics processor is coupled to graphicsmemory 430 which includes several bridge registers including top,bottom, head and tail of the In queue, top, bottom, head and tail of theOut queue, and an In buffer and an Out buffer with counts for eachbuffer. The graphics processor is also coupled to adapter 1 440 andadapter 2 445 across VTC-SA 450.

The Out queue in main memory contains messages or data being sent fromthe main processor to one of the adapters on the VTC-SA. The In queuecontains messages to the main processor being sent from one of theadapters on the VTC-SA. The bridge registers in graphics memory includepointers to the In queue and Out queue in main memory. The In buffer andOut buffer in graphics memory are for shipment of packets of a messagebetween the main processor on the adapters.

FIG. 10A is a flowchart illustrating the use of the queueing system toprovide messages or data from the main processor to one of the adapters.In a first step 500, the main processor loads a message into the Outqueue in main memory and then sends a message directly to the graphicsadapter to update the head register for the Out queue signifying that amessage has been added to the Out queue. In step 505, the graphicsadapter will periodically determine whether the Out buffer in graphicsmemory is full by checking the Out buffer count. If it is full, then thegraphics adapter processor will again periodically poll the bufferperiodically to determine whether data can be entered into the Outbuffer. If the Out buffer is not full, then at step 510 the graphicsprocessor will determine whether there is more data in the Out queue inmain memory by determining whether the head pointer for the Out queue isequal to the tail pointer address plus one. If yes, then there is nomessage remaining in the Out queue in main memory. If there is remainingdata in the Out queue, then the graphics adapter can read via the systembus the data from the Out queue and load it into the Out buffer therebyincreasing the Out buffer count. In addition, the graphics adapter willupdate the tail register for the Out queue to indicate that that portionoff the Out queue has been read.

In step 520, an adapter will periodically query the graphics processoras to whether there is any messages or data in the Out buffer. If thereis data in the Out buffer, then in step 525 the adapter will read theOut buffer via the graphics processor. If the data in the Out buffer wasfor the adapter that read the Out buffer, then the adapter has completedits processing for that parcel of data. Otherwise, the adapter will sendthat data to the appropriate adapter or will notify the appropriateadapter that the data is available and will provide that data to theappropriate adapter upon request. In either case, processing returns tostep 520 where the adapter will continue to query to the graphicsprocessor to determine whether there is more data in the Out buffer.

FIG. 10B is a flowchart illustrating an adapter sending a message ordata to the main processor using the queueing system. In a first step550, the adapter loads a message or data into the In buffer in graphicsmemory via graphics processor 425. The count of space available for theIn buffer is then decreased appropriately. In step 560, the graphicsadapter will then load the message from the In buffer into the In queuein main memory via main processor 400. The graphics processor will thenupdate the head pointer for the In register as well as the count for theIn buffer.

In step 570, the main processor will periodically poll the graphicsadapter to determine whether a message is waiting in the In queue. Ifthere is not a message waiting in the In queue, then the main processorwill continue to periodically poll the graphics processor In pointers todetermine whether such a message may be waiting. In an alternateembodiment, the main processor may simply wait for a message or aninterrupt from the graphics processor indicating that a message iswaiting. If a message is waiting, then in step 580 the main processorwill read the message from the In queue and then request that thegraphics processor update the appropriate tail register for the In queuein graphics memory.

In alternative embodiments, the bridge registers may include multiple Inand Out buffers, perhaps one or more for each adapter on the bus. Inaddition, multiple In and Out queues may be maintained in main memory,again perhaps one or more for each adapter. Furthermore, a separateregister may be included in graphics memory to indicate the address towhich the data is being sent. In addition, the bridge registers and aprocessor for other processing capabilities may be located in anotheradapter on the VTC-SA. In addition, the bridge itself may be a separateadapter on the VTC-SA.

In the preferred embodiment, one of the adapters on the VTC-SA bus isdesignated as a bus distributor which periodically queries the graphicsadapter to determine whether any messages are awaiting any of theadapters on the VTC-SA. This is preferably set up during initializationof the VTC-SA bus. However, the main processor may periodically requestthat a different adapter become the bus distributor. In addition, amulti-distributor situation maybe utilized where each adapterperiodically queries whether any messages are awaiting in the Out bufferand then notifies the appropriate adapter that such a message isawaiting.

Establishing Bus Data Paths

In the preferred embodiment, the main processor controls what data pathsare set up on the remote VTC or VTC-SA video buses. The VTC and VTC-SAbuses are remote from the main processor because neither the mainprocessor nor the system bus are on the remote bus. That is, the mainprocessor must utilize a bus bridge or the like to communicate to theremote bus. The main processor responds to user or application requestfor each connection between adapters on the video bus therebymaintaining a central control system which is under control of the userapplications.

FIG. 11 is a flowchart illustrating how these data paths are set upusing the preferred embodiment of the invention. In a first step 800,the main processor builds a list of adapters on the video bus duringinitialization. In step 510, the main processor then sets up addressesfor the various adapters on the VTC or VTC-SA bus which may also occurduring initialization. The various adapters on the video bus will thenuse these addresses in communicating with each other. More details ofthe initialization process are described above. In step 820, the mainprocessor receives request from a user or application to enable ordisable a desired data path on the bus. The main processor may alsoreceive this request from one of the adapters on the VTC or VTC-SA bususing the above described process for sending messages. In step 830, themain processor then determines whether or not to enable or disable thedesired data path requested. Various criteria may be used. For example,the VTC or VTC-SA bus may simply not have sufficient bandwidth to handlethe requests. The processor may also establish a prioritization schemebetween the various data streams that are enabled. If the main processordetermines not to enable or disable the data path, then the mainprocessor may communicate that back to the user or application and thenprocessing returns to step 820 for the main processor to await furtherrequests for enabling or disabling requests.

If the processor determines that it will enable or disable a data path,then the main processor notifies the adapter or adapters on the videobus to begin or end their data path using the above described processfor sending messages. The main processor may provide only a limited timelength or other limitation on the enabled data path. For example, themain processor may request that a video adapter provide a ten secondvideo display of an incoming video signal. In addition, the mainprocessor may request a video adapter to freeze an incoming video signalonto a given frame until the adapter receives further instructions. Instep 850, the main processor may then notify the graphics adapter thatthe data stream is incoming. If the graphics adapter is always a slaveon the video bus, then such a notification may not be necessary.However, such a notification may allow the graphics adapter toanticipate the incoming data stream and make any appropriate actionsthat would assist such a data stream to occur.

The video adapters then continue processing data and providing datastreams to the graphics adapter or to each other until such time as theincoming data comes to an end, or until the processor requests that theysuspend or end data processing.

Data Formats

This specifies the data format layer of the VTC and VTC-SAarchitectures. The method for determining the Field F1 bits to identifya specific format is bits (24-27) indicate the number of bits per pixel,from a list beginning (1, 2, 4, 8, 12, 16, 24, 32) and the value of bits(24-27) starting with 1. Therefore RGBa32 has an `8` or `1000`b. Largerpixel sizes may be allocated. Bits (28-31) indicate the type of pixelformat, where RGB is type `0` and Index is type `1`. Other types may beallocated. The before described formats are illustrated in FIGS.12A-12B.

RGBalpha32

This data format is described below with reference to FIG. 12A. Thefield bits are preferably as field F1 bits (24-31)=`1000 0000` with 8bits each of Red, Green, Blue, and alpha. 32 bit words contain one RGBapixel as shown. Red, Green and Blue are gamma corrected (non-linear)values expressed as 8 bit binary unsigned integers ranging from 0 to255. Alpha is a blending parameter, such that on completion of a writetransfer the destination pixel contains

alpha* (transferred data value)+(1-alpha)* (value in `other` buffer)

where the `other` buffer is a different frame buffer assumed by theslave and the pixel address in this other buffer is the same as thatused for the write transfer. Slaves need not perform alpha blending. Inread transfers alpha is not defined and slaves must write `0`.

RGB16

This data format is described below with reference to FIG. 12B. Thefield bits are preferably field F1 bits (24-31)=`0110 0000` with 5 bitsRed, 6 bits Green, 5 bits Blue. 32 bit words contain two RGB16 pixels.The two pixels in one 32-bit word are in the order that they wouldappear on a display from left to right, with the left-more pixelstarting at bit 0 in the VTC data word and the right-more pixel startingat bit 16.

RGB8

This data format is described below with reference to FIG. 12C. Thefield bits are preferably field F1 bits (24-31)=`0100 0000` with 3 bitsRed, 3 bits Green, 2 bits Blue. 32 bit words contain four RGB8 pixels asshown. The four pixels in one 32-bit word are in the order that theywould appear on a display from left to right, with the left-most pixelstarting at bit 0 in the VTC data word and the right-most pixel startingat bit 24.

Index8

This data format is described below with reference to FIG. 12D. Thefield bits are preferably field F1 bits (24-31)=`0100 0001` with 8 bitsof palette index. Palette contents are not specified by the VTC. Theyshould be specified or agreed to by other means. 32 bit words containfour Index8 pixels. The order of pixels (0, 1, 2, 4) above is the orderin which they would appear on a display from left to right. While RGB8may in fact be implemented as a specific value of Index8 in a graphicsadapter, RGB8 defines a specific encoding of the bits, and a specificsetting of the palette if a palette is used. Index8, on the other hand,does not inherently imply any particular palette setting. Therefore useIndex8 does not by itself ensure proper interpretation of data valueswhen presented on screen.

Data formats, that is, syntax and semantics, are necessary forconsistent operation in three areas of VTC-SA operation. First, inpackets including Flag and Descriptor words, length and otherconstraints. Second, in commands to VTC devices and their responses.Third, in messages between multiple VTC devices. The compressed data toVTC devices need not be specified here. Such formats are dependent onthe standard being coded or decoded and in some cases on the way inwhich the CODEC system is constructed and configured.

PACKETS

Packets preferably consist of two header words, Flag and Descriptor,followed by a number of data words. The number of words contained isindicated in the Descriptor word. A word, for purposes of thisspecification, is 32 bits. A Packet contains either data or one or morecommands or responses. Data are not mixed in the same packet withcommands and responses.

The syntax of a packet is flag, descriptor, and data contents (thenumber of words specified by the Length field of Descriptor). The Flag,Descriptor, and Data portions of packets are defined as follows. Thevalue of the Flag word may be 0×000001FF. The descriptor bits are:

bit 0: Data/-Command: A value of `1` indicates that the following packetcontains data, e.g. compressed video. A value of `0` indicates that thepacket contains a command or sequence of commands if the packet is beingsent out to a VTC device. A value of `0` indicates that the packetcontains a message or a response to a previously issued command if thepacket is being sent in from a VTC device.

bits 1-3 and 8-15 reserved: devices and tasks producing packets write`0`. Receivers of data ignore the value.

bits 16-31: Length: This field contains an unsigned integer specifyingthe number of words (32 bits each) of data following this word beforethe next Flag word. A value of zero indicates that the next word is aFlag word.

Packet data consists of a number of words of data as specified in theLength field. From the VTC interface the first byte in each word isleft-most, starting at bit 0. I.e. if the queue start is at a loweraddress than the queue end and the bytes of data are added atincrementally higher addresses, then the lower address bytes within eachword are mapped to the left part of VTC words. In 16 bit VTCimplementations the left half-word is transmitted before the right half.

A limit on the size of packets to be transmitted is needed in mostapplications for such reasons as buffer management in the coding anddecoding processes and latency of multiple enqueued processes. Longerpackets in one process mean more time without packets in all otherprocesses, increasing buffer demands and latency. On the other hand,shorter packets increase overhead. Overhead is most likely to be noticedin the software which parses and constructs the system queues. VTCoverhead due to the 2-word packet header is negligible unless mostpackets approach very small sizes. The optimum packet size for thelargest bandwidth streams will depend on the complete softwarearchitecture and network and DASD protocols used, including their packetsizes.

Whether absolute packet size limitations, or a formula for producingapplication dependent limitations, is needed in this architecture isTBD. Whether specified as part of VTC-SA or not, careful attention mustbe paid to packet sizes for reasons of latency.

Conclusion

Although the present invention has been fully described above withreference to specific embodiments, other alternative embodiments will beapparent to those of ordinary skill in the art. Future requirements maydictate the need for greater bandwidth and more devices, with perhapsmore or less inter-device communications. Simultaneously differenttechnologies may become available which would allow i.e. greater clockspeeds for more bandwidth for a given number of data lines. MultipleVTCs with bridges may be implementable, giving effective throughputmultiplied by a factor equivalent to the number of VTCs. A bridge to aninter-box bus, e.g. using optics, may be produced. The logicaldefinition contained here is suitable, as far as we know, for such anextension. As technology progresses, in parallel with the demands on VTCor VTC-SA, the use of electrical logic levels common today may becomeobsolete. Future low-voltage electrical signal levels or optical signalsare clearly possible, yet clearly not plug-compatible with today'ssignal levels and therefore the early implementations. Therefore, theabove description should not be taken as limiting the scope of thepresent invention which is defined by the appended claims.

What is claimed is:
 1. A method of transmitting frames of video data tobe displayed comprising the steps of:a) receiving at least a portion ofa frame of video data; b) determining whether a buffer has sufficientavailable space to store the received video data; c) storing thereceived video data in the buffer if the buffer has been determined tohave sufficient available space to store the received video data; and d)erasing the received video data and any other portions of the frame ofvideo data when other portions of the frame of video data are receivedif the buffer has been determined to not have sufficient available spaceto store the received video data such that the frame of video data isnot displayed even if buffer space is available for the other portionsof the frame of video data.
 2. The method of claim 1 wherein the step ofstoring includes storing the received video data in a queued buffer andupdating a pointer to the queued buffer.
 3. The method of claim 2wherein the step of erasing includes moving the pointer to a location inthe queued buffer prior to the received video data.
 4. The method ofclaim 3 further comprising a step of displaying the received video datastored in the buffer.
 5. The method of claim 4 further comprising a stepof transmitting the received video data to a display adapter prior todisplay of the received video data.
 6. The method of claim 5 furthercomprising a step of digitizing the received video data.
 7. An apparatusfor transmitting frames of video data to be displayed comprising:a)means for receiving at least a portion of a frame of video data; b)means for determining whether a buffer has sufficient available space tostore the received video data; c) means for storing the received videodata in the buffer if the buffer has been determined to have sufficientavailable space to store the received video data; and d) means forerasing the received video data and any other portions of the frame ofvideo data when other portions of the frame of video data are receivedif the buffer has been determined to not have sufficient available spaceto store the received video data such that the frame of video data isnot displayed even if buffer space is available for the other portionsof the frame of video data.
 8. The apparatus of claim 7 wherein themeans for storing includes means for storing the received video data ina queued buffer and means for updating a pointer to the queued buffer.9. The apparatus of claim 8 wherein the means for erasing includes meansfor moving the pointer to a location in the queued buffer prior to thereceived video data.
 10. The apparatus of claim 9 further comprisingmeans for displaying the received video data that has been stored in thebuffer.
 11. The apparatus of claim 10 further comprising means fortransmitting the received video data to a display adapter prior todisplay of the received video data.
 12. The apparatus of claim 11further comprising means for digitizing the received video data.
 13. Adata processing system for transmitting frames of video data to bedisplayed comprising:a) a processor for processing data; b) a memory forstoring data to be processed; c) a video adapter coupled to saidprocessor including:i) means for receiving at least a portion of a frameof video data; ii) means for determining whether a buffer has sufficientavailable space to store the received video data; iii) means for storingthe received video data in the buffer if the buffer has been determinedto have sufficient available space to store the received video data; andiv) means for erasing the received video data and any other portions ofthe frame of video data when other portions of the frame of video dataare received if the buffer has been determined to not have sufficientavailable space to store the received video data such that the frame ofvideo data is not displayed even if buffer space is available for theother portions of the frame of video data; d) a display for displayingthe received video data that has been stored in the buffer from saidvideo adapter under direction off said processor.
 14. The dataprocessing system of claim 13 wherein the means for storing includesmeans for storing the received video data in a queued buffer and meansfor updating a pointer to the queued buffer.
 15. The data processingsystem of claim 14 wherein the means for erasing includes means formoving the pointer to a location in the queued buffer prior to thereceived video data.
 16. The data processing system of claim 15 furthercomprising means for transmitting the received video data to a displayadapter prior to display of the received video data.
 17. The dataprocessing system of claim 16 further comprising means for digitizingthe received video data.
 18. A computer apparatus for allowing a dataprocessing system to transmit frames of video data to be displayedcomprising:a) means for receiving at least a portion of a frame of videodata; b) means for determining whether a buffer has sufficient availablespace to store the received video data; c) means for storing thereceived video data in the buffer if the buffer has been determined tohave sufficient available space to store the received video data; and d)means for erasing the received video data and any other portions of theframe of video data when other portions of the frame of video data arereceived if the buffer has been determined to not have sufficientavailable space to store the received video data such that the frame ofvideo data is not displayed even if buffer space is available for theother portions of the frame of video data.
 19. The computer apparatus ofclaim 18 wherein the means for storing includes means for storing thereceived video data in a queued buffer and means for updating a pointerto the queued buffer.
 20. The computer apparatus of claim 19 wherein themeans for erasing includes means for moving the pointer to a location inthe queued buffer prior to the received video data.
 21. The computerapparatus of claim 20 further comprising means for displaying thereceived video data that has been stored in the buffer.
 22. The computerapparatus of claim 21 further comprising means for transmitting thereceived video data to a display adapter prior to display of thereceived video data.
 23. The computer apparatus of claim 22 furthercomprising means for digitizing the received video data.
 24. The methodof claim 1 wherein the step of storing includes not storing any othervideo data from the frame of the received video data that has beenerased.
 25. The apparatus of claim 7 wherein the means for storingincludes not storing any other video data from the frame of the receivedvideo data that has been erased.
 26. The data processing system of claim13 wherein the means for storing includes not storing any other videodata from the frame of the received video data that has been erased. 27.The computer apparatus of claim 18 wherein the means for storingincludes not storing any other video data from a frame of the receivedvideo data that has been erased.